Measuring Soil Physical Properties with a Texture Analyser

Two of the most important properties of soil are texture and structure. Unlike the definition of ‘texture’ that describes a sample’s physical properties, soil texture is the relative proportion of sand, silt, and clay, and it affects the soil’s ability to hold onto water and nutrients. Soil structure refers to the aggregation of sand, silt, and clay into blocks of soil in various sizes and shapes. This has a significant influence on pore space in soil, and the ease of passage of water, air and roots.

The assessment of soil texture, structure, and other physical properties, is important for agricultural purposes to determine crop suitability and to predict the response of the soil to environmental and management conditions, such as fertiliser.¹  

Soil physical structure plays an integral role in controlling chemical and biological processes. Soils must be sustainably managed to prevent degradation and the loss or reduction of soil functions, including the provision of healthy and nutritious food. To ensure this is possible, reliable information must be available, particularly information on how different soil management practices impact soils.² Tests carried out to help soil management can generally be grouped into those that measure physical, chemical or biological properties. Rapid manual measurements fulfil a useful function in the field; however, there is also a need for accurate laboratory measurements. The use of a Texture Analyser can be invaluable in measuring physical properties. In the case of soil, measurements are generally focussed on shear strength or shearing resistance rather than a pure tensile or compressive test.

Knowledge of the elastic and plastic behaviours of soil is important for the design of earth structures. The construction of road beds and highways should take into account transient and repeated loads, rather than only considering the ultimate compressive strength or bearing capacity of the soil and other materials used. The properties of compacted soils subjected to repeated loading should be investigated using cycle measurements.

In the agricultural world, bulk density and penetration resistance have both been used successfully to represent the potential for physical root proliferation. Penetration resistance is a direct method for assessing soil strength and resistance to root growth. It is directly influenced by water potential, and the relationship between penetration resistance and water potential changes depending on soil management. A laboratory procedure for measuring penetration resistance should control for this property.³ 

Penetration Resistance

Conditioned and suitably equilibrated cores can be subjected to a penetration resistance measurement. To do this, the Texture Analyser is equipped with a 30-degree conical probe. Indentation measurements are performed a set number of times into the core surface, away from the edge and other test sites.

Penetration resistance is calculated as the ratio between force applied and the vertical projection of cone area, and given in MPa. If the measurement is performed to a set distance, penetration resistance can simply be recorded as the maximum force recorded during penetration.

Alternatively, this measurement may also be made using a cylindrical probe, as seen in this study. Researchers from Jiangsu University have been investigating the effect of soil moisture content and end-effector speed on pick-up force and lump damage for seedling transplanting. Efficient transplanting has been identified as one of the essential steps towards achieving an increased yield in the farm. However, many factors are affecting these processes such as soil moisture content and the speed of pickup. This study was carried out to investigate the effect of different soil moisture content and pickup speeds on pickup force, balance, resistance, and lump damage during transplanting of seedlings. They used their TA.XTplus Texture Analyser to measure the penetration resistance of seedling soil lumps. The results showed that penetration resistance was inversely proportional to the speed and soil moisture content.The pick-up damage and the pick-up speed are directly proportional—nevertheless, the former increased with a decreasing soil moisture content. It can be concluded that for successful auto-transplanting of seedlings the soil condition, the force applied and speed should be taken into consideration. This work will implement an effective seedling-picking performance and basis for the optimal design of end-effectors. Read more

Unconfined Yield Stress

Unconfined Yield Stress Rig

Soil consolidation is the change in volume in response to a change in pressure. When a soil layer is subjected to vertical stress, volume change can take place through rearrangement and fracture of soil grains. Both the magnitude and time taken for a volume change are of interest. The magnitude depends on the soil’s compressibility, the applied stress and the thickness of soil. Soil contains water, which moves more readily than solid particles when the sample is being compressed. This will act to cause water to diffuse away from regions with high pressure first, followed by the soil matrix undergoing its own change in volume.

A one-dimensional compression is a simple imitation of the loading applied to soils in many real environments, as soil far away from edges of a loaded area is subject to vertical strain but much lower horizontal strain. 

The use of the Unconfined Yield Stress method provides a two-step test to test firstly the soil’s compressibility and secondly its unconfined strength. 

Vertical Shear

For a pure shear measurement of soil, a Powder Vertical Shear Rig may be used. During this test, a known mass of soil is transferred to the main body of the rig and compressed to a known force to create a uniform cake of soil. A trapdoor below the soil cake is released, exposing a circular surface of the cake. A probe slightly smaller than the hole then pushes a plug of the soil cake through the bottom of the rig. The soil is put into an almost perfect shear state (where the force is parallel with the soil movement).

The main result from this test is the vertical shear strength – the stress the sample can withstand before the cake is pushed out from the rest of the sample bulk. As a shear stress, the force is parallel to the fracture plane (unlike a tensile fracture, which breaks at around 90° to the load).

The soil’s compressibility may also be measured as a function of the distance moved by the probe during the initial consolidation stage.

The physical properties of soil are important to both the agricultural and land structure sectors. Measurement of a variety of these properties is quick and simple using a Texture Analyser.

¹  https://www.element.com/more-sectors/environmental-testing/soil-testing-services/physical-properties-of-soil-testing

²  FAO. 2020. Soil testing methods – Global Soil Doctors Programme - A farmer-to-farmer training programme. Rome. https://doi.org/10.4060/ca2796en

³ Moebius, Bianca N., et al. "Evaluation of laboratory-measured soil properties as indicators of soil physical quality." Soil science 172.11 (2007): 895-912.

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For more information on how to measure texture, please visit the Texture Analysis Properties section on our website.

The TA.XTplus texture analyser is part of a family of texture analysis instruments and equipment from Stable Micro Systems. An extensive portfolio of specialist attachments is available to measure and analyse the textural properties of a huge range of food products. Our technical experts can also custom design instrument fixtures according to individual specifications.

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